/*
 * Copyright 2002-2022 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.core.io;

import java.net.MalformedURLException;
import java.net.URL;
import java.util.Collection;
import java.util.LinkedHashSet;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;

import org.jspecify.annotations.Nullable;

import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;
import org.springframework.util.ResourceUtils;
import org.springframework.util.StringUtils;

/**
 * Default implementation of the {@link ResourceLoader} interface.
 *
 * <p>Used by {@link ResourceEditor}, and serves as base class for
 * {@link org.springframework.context.support.AbstractApplicationContext}.
 * Can also be used standalone.
 *
 * <p>Will return a {@link UrlResource} if the location value is a URL,
 * and a {@link ClassPathResource} if it is a non-URL path or a
 * "classpath:" pseudo-URL.
 *
 * @author Juergen Hoeller
 * @since 10.03.2004
 * @see FileSystemResourceLoader
 * @see org.springframework.context.support.ClassPathXmlApplicationContext
 */
public class DefaultResourceLoader implements ResourceLoader {

	private @Nullable ClassLoader classLoader;

	private final Set<ProtocolResolver> protocolResolvers = new LinkedHashSet<>(4);

	private final Map<Class<?>, Map<Resource, ?>> resourceCaches = new ConcurrentHashMap<>(4);


	/**
	 * Create a new DefaultResourceLoader.
	 * <p>ClassLoader access will happen using the thread context class loader
	 * at the time of actual resource access (since 5.3). For more control, pass
	 * a specific ClassLoader to {@link #DefaultResourceLoader(ClassLoader)}.
	 * @see java.lang.Thread#getContextClassLoader()
	 */
	public DefaultResourceLoader() {
	}

	/**
	 * Create a new DefaultResourceLoader.
	 * @param classLoader the ClassLoader to load class path resources with, or {@code null}
	 * for using the thread context class loader at the time of actual resource access
	 */
	public DefaultResourceLoader(@Nullable ClassLoader classLoader) {
		this.classLoader = classLoader;
	}


	/**
	 * Specify the ClassLoader to load class path resources with, or {@code null}
	 * for using the thread context class loader at the time of actual resource access.
	 * <p>The default is that ClassLoader access will happen using the thread context
	 * class loader at the time of actual resource access (since 5.3).
	 */
	public void setClassLoader(@Nullable ClassLoader classLoader) {
		this.classLoader = classLoader;
	}

	/**
	 * Return the ClassLoader to load class path resources with.
	 * <p>Will get passed to ClassPathResource's constructor for all
	 * ClassPathResource objects created by this resource loader.
	 * @see ClassPathResource
	 */
	@Override
	public @Nullable ClassLoader getClassLoader() {
		return (this.classLoader != null ? this.classLoader : ClassUtils.getDefaultClassLoader());
	}

	/**
	 * Register the given resolver with this resource loader, allowing for
	 * additional protocols to be handled.
	 * <p>Any such resolver will be invoked ahead of this loader's standard
	 * resolution rules. It may therefore also override any default rules.
	 * @since 4.3
	 * @see #getProtocolResolvers()
	 */
	public void addProtocolResolver(ProtocolResolver resolver) {
		Assert.notNull(resolver, "ProtocolResolver must not be null");
		this.protocolResolvers.add(resolver);
	}

	/**
	 * Return the collection of currently registered protocol resolvers,
	 * allowing for introspection as well as modification.
	 * @since 4.3
	 * @see #addProtocolResolver(ProtocolResolver)
	 */
	public Collection<ProtocolResolver> getProtocolResolvers() {
		return this.protocolResolvers;
	}

	/**
	 * Obtain a cache for the given value type, keyed by {@link Resource}.
	 * @param valueType the value type, for example, an ASM {@code MetadataReader}
	 * @return the cache {@link Map}, shared at the {@code ResourceLoader} level
	 * @since 5.0
	 */
	@SuppressWarnings("unchecked")
	public <T> Map<Resource, T> getResourceCache(Class<T> valueType) {
		return (Map<Resource, T>) this.resourceCaches.computeIfAbsent(valueType, key -> new ConcurrentHashMap<>());
	}

	/**
	 * Clear all resource caches in this resource loader.
	 * @since 5.0
	 * @see #getResourceCache
	 */
	public void clearResourceCaches() {
		this.resourceCaches.clear();
	}


	/**
	 * 根据指定的位置获取资源。
	 * 该方法支持从不同来源（如文件系统、类路径和URL）通过协议解析器检索资源。
	 *
	 * @param location 资源的位置。可以是文件路径、类路径引用或URL。
	 * @return 返回检索到的Resource对象。如果未找到合适的资源，则返回null。
	 * @throws IllegalArgumentException 如果location参数为null。
	 */
	@Override
	public Resource getResource(String location) {
		// 确保位置不为null，以防止非法参数异常。
		Assert.notNull(location, "位置不能为空");

		// 遍历所有协议解析器，尝试解析资源。
		for (ProtocolResolver protocolResolver : getProtocolResolvers()) {
			Resource resource = protocolResolver.resolve(location, this);
			if (resource != null) {
				return resource;
			}
		}

		// 如果位置以"/"开头，则将其作为资源路径解析。
		if (location.startsWith("/")) {
			return getResourceByPath(location);
		}
		// 如果位置以类路径前缀开头，则将其作为类路径资源解析。
		else if (location.startsWith(CLASSPATH_URL_PREFIX)) {
			return new ClassPathResource(location.substring(CLASSPATH_URL_PREFIX.length()), getClassLoader());
		}
		else {
			try {
				// 尝试将位置解析为URL...
				URL url = ResourceUtils.toURL(location);
				// 如果是文件URL，则返回FileUrlResource；否则返回UrlResource。
				return (ResourceUtils.isFileURL(url) ? new FileUrlResource(url) : new UrlResource(url));
			}
			catch (MalformedURLException ex) {
				// 如果不是URL，则将其作为资源路径解析。
				return getResourceByPath(location);
			}
		}
	}


	/**
	 * Return a Resource handle for the resource at the given path.
	 * <p>The default implementation supports class path locations. This should
	 * be appropriate for standalone implementations but can be overridden,
	 * for example, for implementations targeted at a Servlet container.
	 * @param path the path to the resource
	 * @return the corresponding Resource handle
	 * @see ClassPathResource
	 * @see org.springframework.context.support.FileSystemXmlApplicationContext#getResourceByPath
	 * @see org.springframework.web.context.support.XmlWebApplicationContext#getResourceByPath
	 */
	protected Resource getResourceByPath(String path) {
		return new ClassPathContextResource(path, getClassLoader());
	}


	/**
	 * ClassPathResource that explicitly expresses a context-relative path
	 * through implementing the ContextResource interface.
	 */
	/**
	 * ClassPathContextResource 类扩展了 ClassPathResource，并实现了 ContextResource 接口。
	 * 它用于表示类路径中的资源，并提供了一些额外的功能，以便在特定的类加载器上下文中操作这些资源。
	 */
	protected static class ClassPathContextResource extends ClassPathResource implements ContextResource {

	    /**
	     * 构造一个新的 ClassPathContextResource 实例。
	     *
	     * @param path 资源的路径。
	     * @param classLoader 可选的类加载器，用于加载资源。如果为 null，将使用默认的类加载器。
	     */
	    public ClassPathContextResource(String path, @Nullable ClassLoader classLoader) {
	        super(path, classLoader);
	    }

	    /**
	     * 获取资源在上下文中的路径。
	     *
	     * @return 资源的路径。
	     */
	    @Override
	    public String getPathWithinContext() {
	        return getPath();
	    }

	    /**
	     * 根据相对路径创建一个新的资源实例。
	     *
	     * @param relativePath 相对路径。
	     * @return 一个新的 Resource 实例，表示相对路径所指的资源。
	     */
	    @Override
	    public Resource createRelative(String relativePath) {
	        // 计算相对路径在当前资源路径下的实际路径。
	        String pathToUse = StringUtils.applyRelativePath(getPath(), relativePath);
	        // 使用计算出的路径和当前类加载器创建一个新的 ClassPathContextResource 实例。
	        return new ClassPathContextResource(pathToUse, getClassLoader());
	    }
	}

}
